Polypropylene reinforced with cellulose whiskers

Polypropylène renforcé par des trichites de cellulose

¹ Centre d’Etudes et de Recherche sur les Macromolécules Végétales, (CERMAV-CNRS- BP53), Grenoble
² Groupe d’Etudes de Métallurgie Physique et de Physique des Matériaux, (GEMPPM- UMR CNRS 5510), INSA, Villeurbanne

Abstract

Since a few years, new nanocomposites materials reinforced with cellulose whiskers have been synthesised in our laboratory. Cellulose whiskers are crystallites in the form of slender rods with lateral size of around 15 nm, a high aspect ratio of about 100 and an important surface area (around 150m²/g). These nanocomposites present spectacular improvement of mechanical properties above the glass rubber transition temperature. This reinforcement was attributed to the formation of a whisker network linked by hydrogen bonds. So far, this type of nanocomposites was processed only with polymer latex, which strongly restricts their applications. Additionally, the role of the interface between whiskers and matrix and the role of the dispersion of whiskers inside the matrix are still not well understood.

In this work, we choose polypropylene (atactic or isotactic), a widely used thermoplastic polymer as a matrix. In an effort to understand the role of interface and dispersion of fillers, we tried to separate these effects. In a first step we prepared different nanocomposites materials reinforced with three different kinds of fillers. In the first kind of suspension, whiskers are homogeneously dispersed. In the second one, they are roughly dispersed and in the last one they are roughly dispersed and grafted by maleated polypropylene.

For atactic polypropylene (aPP) the presence of a low amount of whiskers leads to a great enhancement of the modulus above the glass rubber transition. However, we noticed that for atactic polypropylene, the mechanical properties at low deformation are only slightly different depending on the whiskers dispersion and surface energy. For the isotactic polypropylene (iPP), the polymer morphology observed by optical microscopy is completely different depending on the whisker dispersion and on their surface energy.

Résumé

Les matériaux composites étudiés au cours de ce travail sont composés d’une phase matrice de polypropylène et d’une phase renfort de trichites de cellulose.

Ces trichites de cellulose sont élaborés à partir de l’enveloppe externe d’animaux marins, les tuniciers. Ils présentent une grande cristallinité (95 %), un facteur déformé de l’ordre de 100 et une surface spécifique d’environ 150 m²/g. L’effet de renfort observé après la transition vitreuse est bien supérieur à celui que l’on observe pour des renforts à fibres courtes classiques.

Ce renfort peut être attribué à un phénomène de percolation des trichites au sein de la matrice mais aussi à la création d’une interface forte entre le renfort et la matrice. Pour pouvoir juger de l’importance relative de ces deux contributions, nous avons modifié l’état de surface des trichites (et donc l’état d’interface) et leur état de dispersion au sein de la matrice.